1. Field of the Invention
This invention relates generally to the combustion system of a gas turbine engine and, more particularly, to a double annular combustor having concentrically disposed inner and outer annular combustors with inner and outer domes and a centerbody disposed between the inner and outer domes constructed of a plurality of substantially rectangular segments positioned such that circumferential gaps between adjacent centerbody segments align alternatively with and between carburetors positioned in the inner and outer domes.
2. Description of Related Art
Efforts to reduce emissions in gas turbine engines have brought about the use of staged combustion techniques wherein one burner or set of burners is used for low speed, low temperature conditions such as idle, and another, or additional, burner or burners are used for high temperature operating conditions. One particular configuration of such a concept is that of the double annular combustor wherein the two stages are located concentrically in a single combustor liner. Conventionally, the pilot stage section is located concentrically outside and operates under low temperature and low fuel/air ratio conditions during engine idle operation. The main stage section, which is located concentrically inside, is later fueled and cross-ignited from the pilot stage to operate at the high temperature and relatively high fuel/air ratio conditions. The swirl cups of the respective pilot and main stage sections generally lie in the same radial and circumferential planes, as exemplified by U.S. Pat. No. 4,292,801 to Wilkes, et al. and U.S. Pat. Nos. 4,374,466 and 4,249,373 to Sotheran.
However, as discussed in a development report to the National Aeronautics and Space Administration (NASA) on combustion system component technology for the Energy Efficient Engine (E.sup.3) and U.S. Pat. No. 4,194,358 to Stenger, the pilot stage and the main stage may be radially offset (i.e., lie in distinct radial planes). In both the '358 patent and E.sup.3 configurations, the effective length of the main stage section is relatively short and the effective length of the pilot stage section is relatively long. This configuration allows for complete or near-complete combustion to reduce the amount of hydrocarbon and carbon monoxide emissions since there is a relatively long residence time in the pilot stage section and a relatively minimal residence time in the main stage section.
Whether the inner and outer combustors are radially aligned or not, and whether the outer annular combustor acts as the pilot stage or main stage, the prior art discloses the use of a centerbody to isolate the pilot and main stages. The intended purpose of isolating the pilot stage from the main stage is to ensure combustion stability of the pilot stage and lower gaseous emissions at various operating points.
Combustor centerbodies generally have been a continuous ring fabricated from forged or rolled rings and sheet material. This one-piece design, however, is difficult to manufacture due to tight size and form tolerance requirements for fabrication and assembly. Further, the difference in temperature between the combustor structure and the centerbody, as well as between individual points within the centerbody structure, generate large hoop stresses and associated forces at the point of attachment. Another problem with one-piece centerbodies is the effect on the entire piece caused by a local problem. For example, the entire centerbody is depressurized in the event of a local burn-through due to the resulting leakage. Also, if one area of the centerbody is damaged the entire piece must be repaired or replaced.
It is also understood that replacement of a one-piece centerbody with a centerbody having a plurality of segments introduces other considerations. In particular, freedom of movement for such segments is required during various staging modes of a double annular combustor since each centerbody segment experiences an axial thermal gradient from the relatively cold upstream end and the hot downstream end, as well as a circumferential thermal gradient from one side to the other due to inherent hot streaks from the fuel nozzles positioned within the carburetors. Therefore, such thermal gradients must be accounted for in addition to the radial thermal gradients already present on one-piece centerbodies. Accordingly, a combustor centerbody having a plurality of segments is needed that eliminates the problems associated with one-piece centerbodies and accounts for the radial, axial and circumferential thermal gradients which arise therefrom.